In big fights, where I send a single champion ship to smash a defense fleet, I have no problem using all 20 shots from multiple blasters. Occasionally, even 30 (+10 from Expanded Magazines) is not enough. Expanded Magazines is already useful for heavy Antimatter Blaster use. Antimatter blaster is one of the most expensive light weapons in the game to mount. It had better be worth the OP spent.
EDIT - I see, you use a mod ship that can mount several. The most blasters I would mount on a standard ship (and focus fire all of them at one target) would be six on a Doom. For smaller ships, I can mount up to four at most. Even with Expanded Magazines, 120 shots from four blasters is not enough to defeat a defense fleet.